- Compare atomic structure, physical state, oxidising power and the strength of the HX acids from F to I.
- Write equations for making chlorine in the laboratory and in industry and for its reactions with water and with cold and hot alkalis, and calculate from them.
- Know the properties and names of hydrogen chloride, hydrochloric acid and the oxyacids of chlorine.
- Identify halide ions with AgNO₃ and decide which halogen displacement reactions take place.
The sharp smell of pool water, laundry bleach, the acid in gastric juice, iodised salt and the fluoride in toothpaste all come from one family. Halogen is Greek for «salt-former»: these elements form salts directly with metals (2Na + Cl₂ → 2NaCl). The 2026 DİM exams had three tasks on halogens: making chlorine water, metals reacting with hydrochloric acid and comparing the halogens.
General properties of the halogens
The halogens are the elements of group VIIA: fluorine F, chlorine Cl, bromine Br, iodine I (and radioactive astatine At). Their outer shell holds 7 electrons (ns²np⁵): one short of an octet, so they are typical non-metals and strong oxidising agents. Their elements consist of diatomic molecules (F₂, Cl₂, Br₂, I₂), and in nature halogens occur only in compounds. The electron configuration of chlorine is 1s²2s²2p⁶3s²3p⁵.
| Element | Outer shell | Element (room conditions) | Oxidation states |
|---|---|---|---|
| F | 2s²2p⁵ | pale yellow gas | −1 |
| Cl | 3s²3p⁵ | yellow-green gas | −1, +1, +3, +5, +7 |
| Br | 4s²4p⁵ | red-brown liquid | −1, +1, +3, +5, +7 |
| I | 5s²5p⁵ | dark violet crystals that sublime | −1, +1, +3, +5, +7 |
- F → I: atomic radius, relative atomic mass, melting and boiling points increase (gas → liquid → solid).
- Electronegativity and oxidising power decrease (F is the strongest oxidising agent), while the reducing power of X⁻ ions increases (I⁻ is the strongest reducing agent).
- The H–X bond weakens and acid strength increases: HF is weak, HI the strongest.
- Fluorine has no d-sublevel and shows only −1. In excited states Cl, Br and I have 3, 5 or 7 unpaired electrons: hence +3, +5, +7.
Masses of AgNO₃ that react completely with equal masses (a g) of the salts:
NaCl — a g — m₁
NaI — a g — m₂
NaBr — a g — m₃
Choose the correct relation between m₁, m₂ and m₃. Mr(NaCl) = 58.5, Mr(NaBr) = 103, Mr(NaI) = 150
A) m₁ = m₂ = m₃ B) m₂ > m₃ > m₁ C) m₁ > m₃ > m₂ D) m₃ > m₁ > m₂ E) m₁ > m₂ > m₃
Show solutionHide solution
So n(AgNO₃) = n(NaX) = a / M: the smaller the molar mass of the salt, the more AgNO₃ is used.
M: NaCl (58.5) < NaBr (103) < NaI (150) → m₁ > m₃ > m₂.
Answer: C.
Preparing chlorine
In the laboratory chlorine is made by oxidising concentrated hydrochloric acid with strong oxidising agents (Cl⁻ → Cl₂):
MnO₂ + 4HCl → MnCl₂ + Cl₂↑ + 2H₂O (on heating)
2KMnO₄ + 16HCl → 2KCl + 2MnCl₂ + 5Cl₂↑ + 8H₂O
K₂Cr₂O₇ + 14HCl → 2KCl + 2CrCl₃ + 3Cl₂↑ + 7H₂O
KClO₃ + 6HCl → KCl + 3Cl₂↑ + 3H₂O
In industry chlorine is made by electrolysis of NaCl solution: 2NaCl + 2H₂O → 2NaOH + H₂↑ + Cl₂↑ (Cl₂ forms at the anode, H₂ at the cathode). Electrolysis of the melt also gives sodium metal: 2NaCl → 2Na + Cl₂↑.
- n(oxidant)amount of the oxidising agent, mol
- zelectrons gained by one formula unit: MnO₂ → 2, KMnO₄ → 5, K₂Cr₂O₇ → 6
Chlorine from oxidising concentrated HCl (when the oxidant contains no chlorine). KClO₃ is the exception: its own chlorine also ends up in Cl₂, 1 mol of KClO₃ → 3 mol of Cl₂.
1) 17.4 g of MnO₂ was heated with excess concentrated hydrochloric acid. What volume of chlorine (STP) forms? How many moles of the HCl used acted as the reducing agent? Mr(MnO₂) = 87
2) What volume of chlorine (STP) can be obtained from 15.8 g of KMnO₄? Mr(KMnO₄) = 158
Show solutionHide solution
By the equation 0.2 mol of MnO₂ uses 0.8 mol of HCl; half of it — 0.4 mol — is oxidised to Cl₂ (the reducing agent), the other 0.4 mol ends up in MnCl₂.
2) n(KMnO₄) = 15.8 / 158 = 0.1 mol; Mn⁺⁷ → Mn⁺², z = 5: n(Cl₂) = 0.1 · 5 / 2 = 0.25 mol → V = 5.6 L.
Chemical properties of chlorine
Chlorine is a strong oxidising agent. With metals it forms chlorides, and metals with variable valency are oxidised to their highest oxidation state: 2Na + Cl₂ → 2NaCl, 2Fe + 3Cl₂ → 2FeCl₃, Cu + Cl₂ → CuCl₂. In light it combines with hydrogen explosively: H₂ + Cl₂ → 2HCl. With phosphorus it gives PCl₃ and PCl₅, but it does not react directly with oxygen, nitrogen or carbon. It also oxidises compounds: 2FeCl₂ + Cl₂ → 2FeCl₃, H₂S + Cl₂ → S↓ + 2HCl.
In reactions with water and alkalis chlorine is both the oxidising and the reducing agent: some of its atoms become Cl⁻¹ and others Cl⁺¹ or Cl⁺⁵.
- Cl₂ + H₂O ⇌ HCl + HClO — chlorine water. Hypochlorous acid decomposes: HClO → HCl + [O]; the atomic oxygen bleaches dyes and kills microbes. In light: 2HClO → 2HCl + O₂↑.
- Cold alkali: Cl₂ + 2NaOH → NaCl + NaClO + H₂O (bleach solution).
- Hot alkali: 3Cl₂ + 6KOH → 5KCl + KClO₃ + 3H₂O (Berthollet’s salt KClO₃).
- Dry slaked lime: Cl₂ + Ca(OH)₂ → CaOCl₂ + H₂O (bleaching powder, used to bleach and disinfect).
A more active halogen freeing a less active one from a solution of its salt: Cl₂ + 2KBr → 2KCl + Br₂, Cl₂ + 2KI → 2KCl + I₂, Br₂ + 2KI → 2KBr + I₂. Iodine cannot displace any of them. Fluorine in solution reacts with the water rather than with the halide: 2F₂ + 2H₂O → 4HF + O₂.
3Cl₂ + 6KOH → 5KCl + KClO₃ + 3H₂O
Choose the true statements about this reaction.
1. Chlorine is both the oxidising and the reducing agent
2. Berthollet’s salt forms
3. The chlorine atoms acting as reducing and as oxidising agent are in the ratio 5 : 1
4. The reaction takes place in the cold
5. In one of the products chlorine has the oxidation state +5
Show solutionHide solution
2 is true: KClO₃ is Berthollet’s salt; 5 is true: chlorine is Cl⁺⁵ in KClO₃.
3 is false: reducing : oxidising = 1 : 5.
4 is false: chlorate forms in hot alkali, hypochlorite in the cold.
Answer: 1, 2, 5.
Which reactions take place?
I. Br₂ + 2NaCl →
II. Cl₂ + 2NaI →
III. I₂ + 2KBr →
IV. Br₂ + 2KI →
A) I, II B) II, IV C) I, III D) III, IV E) II, III
Show solutionHide solution
II: Cl₂ + 2NaI → 2NaCl + I₂ — goes. IV: Br₂ + 2KI → 2KBr + I₂ — goes.
I: bromine is weaker than chlorine; III: iodine is weaker than bromine — no reaction.
Answer: B.
Hydrogen chloride and hydrochloric acid
Hydrogen chloride HCl is a colourless gas with a sharp smell, heavier than air and extremely soluble: at 0 °C one volume of water dissolves about 500 volumes of HCl. Its solution is hydrochloric acid; the concentrated acid contains about 37% HCl by mass and «fumes» in air. In the laboratory concentrated sulfuric acid is added to solid table salt: NaCl + H₂SO₄ → NaHSO₄ + HCl↑ (gentle heating; with strong heating 2NaCl + H₂SO₄ → Na₂SO₄ + 2HCl↑). In industry HCl is made by burning hydrogen in chlorine and dissolving the gas in water.
Hydrochloric acid is a strong monobasic acid: it releases hydrogen with metals before hydrogen (Zn + 2HCl → ZnCl₂ + H₂↑; iron gives FeCl₂) and does not react with Cu, Ag or Hg; it neutralises basic oxides and bases and decomposes salts of weak and volatile acids: CaCO₃ + 2HCl → CaCl₂ + CO₂↑ + H₂O, FeS + 2HCl → FeCl₂ + H₂S↑. Gastric juice contains hydrochloric acid too. Its salts are chlorides: NaCl (table salt), KCl (fertiliser), CaCl₂ (drying agent), AgCl.
- n(Me)amount of metal, mol
- vvalency of the metal in its chloride (Na → 1, Mg, Zn, Fe → 2, Al → 3)
Metal + HCl: one mole of metal uses as many moles of HCl as its valency and releases half as many moles of H₂
1) Equal masses of which metal release the most hydrogen with excess hydrochloric acid?
A) Mg B) Zn C) Fe D) Al E) Ca
2) When 20 g of an Fe and Cu mixture was treated with excess hydrochloric acid, 4.48 L of hydrogen (STP) was released. Calculate the mass fraction (%) of copper in the mixture and the mass (g) of HCl used. Ar(Fe) = 56, Mr(HCl) = 36.5
Show solutionHide solution
2) Copper does not react with HCl; only iron releases hydrogen: Fe + 2HCl → FeCl₂ + H₂↑.
n(H₂) = 4.48 / 22.4 = 0.2 mol → n(Fe) = 0.2 mol, m(Fe) = 0.2 · 56 = 11.2 g.
m(Cu) = 20 − 11.2 = 8.8 g → w(Cu) = 8.8 / 20 · 100% = 44%.
n(HCl) = 2 · 0.2 = 0.4 mol → m = 0.4 · 36.5 = 14.6 g.
Oxyacids of chlorine. Tests for halide ions
| Acid | Oxidation state of Cl | Name | Salts |
|---|---|---|---|
| HClO | +1 | hypochlorous acid | hypochlorites: NaClO, Ca(ClO)₂ |
| HClO₂ | +3 | chlorous acid | chlorites: NaClO₂ |
| HClO₃ | +5 | chloric acid | chlorates: KClO₃ |
| HClO₄ | +7 | perchloric acid | perchlorates: KClO₄ |
Berthollet’s salt KClO₃ decomposes on heating: with a catalyst (MnO₂) 2KClO₃ → 2KCl + 3O₂↑ — a laboratory way to make oxygen; without a catalyst 4KClO₃ → 3KClO₄ + KCl. Match heads contain KClO₃ as well. Halide ions are detected with AgNO₃: Ag⁺ + Cl⁻ → AgCl↓ (white, curdy), Ag⁺ + Br⁻ → AgBr↓ (pale yellow), Ag⁺ + I⁻ → AgI↓ (yellow). AgF is soluble, so the F⁻ ion is detected with Ca²⁺: Ca²⁺ + 2F⁻ → CaF₂↓. Free iodine turns starch blue.
- Uses: chlorine — water disinfection, HCl, PVC and bleaches; fluorine — Teflon, toothpaste; HF etches glass (SiO₂ + 4HF → SiF₄↑ + 2H₂O); AgBr — photography; 5% tincture of iodine, iodised salt.
- In nature: chlorine — NaCl (rock salt, sea water) and KCl, fluorine — fluorite CaF₂; in Azerbaijan iodine and bromine are obtained from oil-field formation waters.
Adding AgNO₃ to a solution of salt X gives a yellow precipitate. Adding chlorine water to a solution of X turns it brown, and then starch turns it blue. Identify X.
A) KCl B) KBr C) KI D) KF E) KNO₃
Show solutionHide solution
KBr gives pale yellow AgBr, but bromine does not turn starch blue; KCl gives a white precipitate; KF and KNO₃ give no precipitate with AgNO₃.
Answer: C) KI.
Key points
- Halogens (ns²np⁵) are strong oxidising agents; from F to I the radius grows, oxidising power falls and the HX acids get stronger.
- In the laboratory Cl₂ is made by oxidising concentrated HCl with MnO₂, KMnO₄, K₂Cr₂O₇ or KClO₃; in industry by electrolysis of NaCl solution.
- With water and alkalis chlorine is both oxidising and reducing agent: chlorine water (HCl + HClO), cold alkali — hypochlorite, hot alkali — chlorate (KClO₃), slaked lime — CaOCl₂.
- An active halogen displaces a less active one from its salt (Cl₂ > Br₂ > I₂); iron gives FeCl₃ with chlorine and FeCl₂ with hydrochloric acid.
- HClO, HClO₂, HClO₃, HClO₄ — hypochlorous, chlorous, chloric, perchloric acids, getting stronger. Halides with AgNO₃: AgCl white, AgBr pale yellow, AgI yellow.
Check yourself
12 questions. Every correct answer earns XP.